7. PROJECT SUMMARY / ABSTRACT Periodontitis is a chronic inflammatory disease that is the leading cause of periodontal tissue destruction and tooth loss. The cost of treating periodontal diseases in the U.S. in 2011 was estimated at $50 billion. Severe periodontitis has been associated with Porphyromonas gingivalis (Pg) colonization of the oral mucosa and has been characterized as an immune-driven disease. The impact of mucosal oral Langerhans cells (oLCs) on the differentiation of pathogenic CD4+ T helper cells (Th) and alveolar bone destruction remains unresolved. Our current objective is to determine how oLCs drive a destructive Th17 response in the mouse periodontitis model. We hypothesize that oLCs produce a toll-like receptor/MyD88-dependent diffusible signal that conditions underlying lamina propria-resident dendritic cells (DCs) to a Th17 polarizing program in a manner independent of major histo-compatibility class II (MHC-II)-restricted antigen presentation by oLCs. To test our hypothesis we will pursue two specific aims using a novel antigen-specific tetramer to determine the kinetics of activation and phenotype of Pg-specific Th and regulatory CD4+ T cells in two mutant mouse strains. Aim 1: Determine the requirement of Ag:MHC-II presentation in oLCs to drive differentiation of antigen-specific Th17 cells after oral colonization of mice with Pg. We hypothesize that oLCs indirectly influence the polarization of nave CD4+ T cells by locally ?educating? lamina propria dendritic cells in the mucosae towards a polarization program that differentiates Th17 cells. We predict that cognate MHC-II:T cell receptor interaction between oLCs and nave CD4+ T cells is not required to drive polarization by lamina propria dendritic cells. Mice unable to present microbial MHC-II peptides on oLCs are, therefore, expected to differentiate a normal Th17 type response to Pg. Aim 2: Determine the requirement of MyD88-dependent signaling in oLCs to drive differentiation of antigen-specific Th17 cells after oral colonization of mice with Pg. We hypothesize that MyD88- dependent pathogen recognition signaling induces oLCs to express bystander inflammatory signal(s). These signals condition lamina propria dendritic cells to drive a Th17-biased polarization program. We predict that lamina propria dendritic cells will be unable to drive Th17 polarization in mice lack TLR/MyD88-dependent signals on oLC. The expected scenario in MyD88-deficient oLC mice will be a Th1 polarization when encountering Pg at mucosal surfaces. This Th1-differentiation scenario is expected to be identical to what we have discovered in mice that lack oLCs. Data from this R03 funding mechanism will support an R01 application to define the mechanism by which oLCs drive local conditioning of lamina propria dendritic cells. By understanding the role played by oLCs in directing periodontal disease comes the opportunity to manipulate this arm of immunity to achieve better disease outcomes through vaccine targeting or by using immunotherapy to interfere with signals emanating from oLCs.